Various kinds of combustible materials are contained in a secondary battery. As a result, the secondary battery may be heated or explode due to overcharge of the secondary battery, overcurrent in the secondary battery, or other external physical impact applied to the secondary battery. That is, the safety of the secondary battery is very low. Consequently, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), to effectively control an abnormal state of the secondary battery, such as overcharge of the secondary battery or overcurrent in the secondary battery, are disposed on a battery cell in a state in which the safety elements are connected to the battery cell.
Meanwhile, a secondary battery has a tendency for the thickness of the secondary battery to be increased when the secondary battery is charged and discharged for 300 to 500 cycles as compared with the thickness of the secondary battery before the secondary battery is charged and discharged. For example, after a secondary battery having a thickness of 6 mm is charged and discharged for 300 to 500 cycles, the thickness of the secondary battery is increased by 0.42 to 0.48 mm with the result that the secondary battery has a thickness of 6.42 to 6.48 mm.
Such increase in thickness of the secondary battery results in increase in thickness of a battery pack, including a single battery or a plurality of batteries, which is mounted in an external device. When the battery pack, the thickness of which has been increased, is assembled into a laptop computer or a mobile phone, an assembly process may be difficult or, according to circumstances, the assembly process may not be possible.
Up to now, a method of reducing the thickness of a portion of the inside part of a battery case, which is in contact with the side of an upper case or a lower case of the battery pack extending in the battery thickness direction, so as to compensate such increase in thickness of the battery pack has mainly been used.
The above method partially provides an effect of preventing the change in thickness of the battery pack due to expansion in thickness of the battery cells. Since the battery case is manufactured by plastic injection molding, however, a large quantity of defects, such as flow marks, unmolding, contraction and bending, occur during mass production of the battery case, which is designed so that the thickness of a portion of the battery case is smaller than that of the remaining portion of the battery case, through injection molding.
Therefore, there is a high necessity for a battery pack having a specific structure in which the thickness of a battery case is uniform, and the increase in thickness of the battery pack due to charge and discharge of battery cells is prevented.
Meanwhile, a battery pack mounted in a laptop computer requires high power and large capacity. To this end, a conventional cylindrical battery pack including a plurality of cylindrical battery cells has generally been used. In recent years, however, the size of a laptop computer has been reduced, and therefore, there is a high necessity for a slim type battery pack.
Therefore, there is a high necessity for a technology in which pouch-shaped battery cells are used to manufacture a slim type battery pack, thereby increasing capacity of the battery pack, and a spacer is mounted between a pack case and a battery cell array, thereby securing a space accommodating the increase in thickness of the battery cell array during the charge and discharge of the battery pack.